During the production of oil country tubulars the preparation for the threaded ends of seamless pipe is to first anneal the pipe end by heating it to 400 deg C. to 705 deg C. for the required length to be formed. The pipe ends are then either swaged to a smaller diameter or expanded to a larger diameter as required by the resulting joint design. To attain the desired final joint properties, the pipe is then annealed a second time prior to threading. Typical heated lengths range from 4″ to 20″ and typical diameters range from 2⅜″ to 18⅝″ with wall thicknesses from 0.205″ to 1.5″. To date there are various methods of heating the pipe ends, but they are very limited as to the variation of heated lengths that can be heated with any given setup, and are to some degree limited in the diameter range that can be heated. A Single Coil Fixed Coil Concept involves a simple solenoid coil generally powered at a higher frequency than 60 Hz, typically 500 Hz to 3 kHz. This coil design has a fixed heated length dependent somewhat on the pipe diameter, and therefore a different coil must be supplied for every significant change in heated length. This is expensive and unproductive. In this case the pipe end temperatures are controlled by the length of coil overhanging the desired heated length.
Another approach is a Fixed Position Diverter Concept using a tapped solenoid coil generally operated at frequencies above 60 Hz to adjust for heated length and a flux diverter to boost the end temperature. This is more flexible than a straight solenoid coil but requires that the operator change taps for every heated length, and as a result, there is substantial loss of production and lack of flexibility.
A further concept is a Coil with Distributed Turns and External Flux Concentrators using a solenoid coil with distributed turns of a fixed length with no taps and external flux concentrators to concentrate the flux at the ends of the coil to the inner diameter of the coil on the pipe insertion and to the centerline on the end of pipe end. This technique is similar to the Fixed Solenoid Coil with coils required for nearly every heated length. Accordingly, no current pipe end heating provides for universal applicability to a variety of different pipe diameters and desired heating lengths without significant system adjustment.